Migration experts have challenged new official projections that Britain's population will hit 70 million by 2029 largely as a result of future migration and births to foreign nationals in the UK.

Tim Finch, head of the Institute of Public Policy Research's migration programme, said the Office of National Statistics projections published today assumed that migration patterns over the next 10 years will mirror those of the last 10 years.

The ONS figures are based on the pattern of migration into Britain up until the middle of 2008 before the economic recession began to bite: "Early indications suggest that the peak of net migration, mainly from eastern Europe, has passed. The next few sets of migration figures will suggest that inward migration is steadying and emigration is increasing," said Finch. "If that trend continues then some of the assumptions that net migration will go on and on that lie behind the projection of 70 million 2029 may be wrong."

The ONS projections show that the current UK population of 61.4 million would rise to 71.6 million by 2033, passing the 70 million mark by 2029.

The official statisticians say 55% of the 10.2 million projected rise in the UK population over the next 25 years will come from a natural increase in births over deaths and that 45% will be due to more people coming to live in Britain than those emigrating.

But these projections are actually lower than the set published last year and are based on what was happening with immigration in the middle of 2008. The ONS has revised net migration downwards by 10,000 to 180,000 a year and projected that the 70 million mark will be passed a year later at 2029 compared with last year's projections.

Phil Woolas, the immigration minister, said today's projections showed that population growth was starting to slow down and the impact of the radical reforms that the Home Office has made to the immigration system over the last two years were working.

"Last year saw a 44% fall in net migration and we expect that fall to be sustained and reflected in future projections.

"Our new flexible points-based system is giving us greater control on those coming to work or study from outside Europe, ensuring that only those that Britain need can come."

Finch said it was still early days for the points-based system introduced over the last year: "We do not yet know the extent to which the PBS is biting."

The ONS also suggests the oldest age group is likely to grow the most quickly with the number of Britons over 85 set to more than double over the next 25 years from 1.3 million in 2008 to 3.3 million by 2033. The numbers of centenarians is set to rise from 11,000 to 80,000 by 2033.

Even taking account of the forthcoming increases in the state pension age, those who qualify for a state pension are set to rise from 11.6 million in 2008 to 15.6 million by 2033.

New life expectancy figures confirm the north-south divide in Britain. Life expectancy is highest for men at 79.2 years in south-east England and for women at 83.1 in south-west England. It is lowest in Scotland, at 75 for men and 79.9 for women.

Life expectancy at birth has improved across the UK since the early 1990s but while London saw an increase of 4.9 years for men, life expectancy for Scottish men rose only by 3.5 years.

A boy born this year in Kensington and Chelsea can expect to live for 84.3 years compared with only 70.7 years for a boy born in central Glasgow.